During the fabrication of semiconductor integrated circuits, materials are formed on the wafer surface and portions of layers must be removed or etched. Materials which are etched include semiconductors, conductors (such as metal) and insulators such as nitrides and oxides. The present invention is concerned primarily with oxides.
Numerous different oxides may be utilized during device fabrication. These oxides may be thermally grown or deposited, for example by a TEOS (tetraethoxysilane) process. Silicate glass such as PSG (phosphosilicate glass) and BPSG (boron phosphosilicate glass) are also used. In addition, undesired oxide films, such as native or chemical oxides, may be formed due to the wafer's exposure to the environment. In many process flows, it is desirable to selectively etch one type of oxide but not another.
The selectivity of a process is a measure of the etch rate of the layer to be etched relative to other materials on the wafer surface. The selectivity of etches to different materials is an important process characteristic. Overetching is the common practice of submitting a layer to the etch process for a time longer than is necessary to just etch through the layer at one part of a wafer. It is because of this overetch that the selectivity to layers underlying the etched layer or to layers exposed simultaneously with the etched layer becomes critical.
Accordingly, there is a need in the industry for an etch process which selectively etches different types of oxides.